Direct injection gasoline engines offer improved efficiency because fuel injected directly into a cylinder can reduce cylinder charge temperature. As a result, additional air may enter a cylinder as compared to an equivalent cylinder that has port injected fuel. Consequently, engine power and efficiency may be improved. In addition, direct injection gasoline engines may exhibit improved transient fuel control because there is less tendency for fuel to collect at a cylinder intake port of a direct injection engine than for a port fuel injection engine. However, direct injection engines may generate soot at higher engine speed and load conditions because there is less time available to atomize fuel in the cylinder. As a result, it may be useful to incorporate a particulate filter in the exhaust system of a direct injection engine. Gasoline engines include those engines fueled by pure gasoline or mixtures of gasoline and other fuels such as alcohols. Further, other fuels used in spark ignited engines are also included such as liquid propane gas (LPG) or compressed natural gas (CNG).
In U.S. Pat. No. 6,738,702 a method for regenerating a particulate filter is described. In particular the reference describes a diesel engine having a particulate filter followed by a lean NOx catalyst or a SCR catalyst. The method describes adjusting engine fuel to provide an air-fuel ratio for purging catalyst. However, the method provides little detail as to how engine fuel is adjusted and any such adjustment is less likely to be effective for gasoline engines because gasoline engines are often operated at stoichiometric conditions whereas diesel engines are often operated with lean air-fuel mixtures.
The inventors herein have developed a method for adjusting fuel supplied to a spark ignited engine, comprising: providing an air-fuel mixture that varies about stoichiometric conditions, said air-fuel mixture substantially stoichiometric on average over a number of cylinder cycles while not actively regenerating a particulate filter during a first operating condition; and providing an air-fuel mixture that varies about stoichiometric conditions, said air-fuel mixture lean of stoichiometric on average over a number of cylinder cycles while actively regenerating said particulate filter during a second operating condition, said second operating condition different from said first operating condition.
By providing an air-fuel mixture that varies about stoichiometric conditions when not regenerating a particulate filter during a first operating condition, and by providing an air-fuel mixture that varies about stoichiometric, the air-fuel mixture lean of stoichiometric conditions during a second operating condition a three-way catalyst may operate efficiently whether or not a particulate filter is regenerating. For example, when a particulate filter is not regenerating little oxygen contained in exhaust gases participates in soot oxidation. On the other hand, during particulate filter regeneration, excess oxygen in exhaust gases may be consumed by oxidation of soot. If the engine is operating with a substantially stoichiometric air-fuel mixture while the particulate filter is not regenerating a three-way catalyst in the engine exhaust system can operate efficiently because there are oxidants and reductants in the exhaust gases. If the engine is operated with an air-fuel mixture that is lean of stoichiometric conditions, the excess oxygen can be consumed by oxidizing soot so that a substantially stoichiometric mixture enters a three-way catalyst for processing. Thus, whether the particulate filter is regenerating or not, the catalyst is exposed to a substantially stoichiometric exhaust gas mixture.
The present description may provide several advantages. Specifically, the description provides a method for storing and regenerating carbonaceous particulate emissions from a spark-ignited engine while maintaining net stoichiometric exhaust conditions to control tailpipe emissions. The approach may improve engine emissions by providing improved control over exhaust gases that are processed by a three-way catalyst. Further, engine air-fuel control may be simplified, especially if an oxygen sensor is placed in the exhaust system between the particulate filter and a downstream catalyst because a direct measurement of the amount of oxygen consumed by oxidizing soot is possible rather than an inference made by a model. Further still, the method regenerates a particulate filter without having to provide additional hardware, such as an air pump.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.